Another novel use of a quantum singularity was in 2378, when the crew of Voyager was able to bounce a tachyon beam off of one and successfully establish communication with Starfleet, on the other side of the galaxy. This communication was limited by the fact that the singularity was only in alignment for eleven minutes every day. (VOY: "Author, Author")

In 2371, the temporal energy being radiated from the artificial quantum singularity of a cloaked Warbird orbitingstationDeep Space 9 interacted with the delta-series radioisotopes which Chief O'Brien had absorbed due to the explosion of a plasma conduit. This had the effect of causing O'Brien to shift forward in time by approximately five hours every time the singularity emitted temporal radiation at a specific wavelength. Later, O'Brien and Doctor Bashir were able to control O'Brien's time shifts by flooding his body with more of the isotopes and calibrating the decay rate to match the temporal frequency of the singularity. (DS9: "Visionary")

A type 4 quantum singularity is a highly localized disturbance in the spacetime continuum, characterized by a gravimetric flux density of over two thousand percent.

In 2371, Voyager encountered and became trapped within the event horizon of a type 4 quantum singularity in the Delta Quadrant. Due to massive temporaldistortions being emitted from the singularity, the crew of Voyager encountered time-delayed reflections of their own ship, initially making them believe that they had encountered another vessel being pulled into the singularity. Eventually, they were able to find their way out by exiting through a breach in the event horizon which they had created when they initially entered. (VOY: "Parallax")

This is probably a black hole, judging by the description of the crew. However, the term was not in the episode.